Manchester Memoirs, Vol. Ixii. (19 18), No. 10. 5 



A number of different specimens were used, including violet, 

 purple, 1 green and yellow crystals from Alston Moor, Cumberland ; 

 a colourless specimen from the same locality ; a pale green chloro- 

 phane from Cornwall ; a colourless specimen from the Pyrenees, 

 and another from Matlock. 



All the coloured specimens without exception lost their colour 

 on heating, but the time and temperature necessary to completely 

 discharge the colour varied considerably. The green specimens 

 were the most difficult to decolourise completely, a dull red heat 

 being necessary for fully five minutes. The colours artificially 

 induced by the action of radium were much more readily dis- 

 charged, warming to a temperature not much above ioo° C. being 

 sufficient in many cases. 



Thermo-luminescence was very marked in all the naturally 

 coloured crystals, a violet light being emitted in most cases, 

 frequently strong enough to read by at a distance of six inches. 

 Decrepitation was generally violent, except with the pale purple 

 crystals, but no naturally coloured crystal was found which could 

 be heated without some decrepitation. The yellow and dark 

 violet varieties were most violent. 



Of the colourless varieties, that from the Pyrenees gave an 

 almost invisible glow when very strongly heated, while that from 

 Matlock gave no light at all. Neither of these showed any tendency 

 to decrepitate. The Alston Moor colourless specimen on the con- 

 trary gave a very lively display of decrepitation and also of beautiful 

 violet light. The Cornish chlorophane also decrepitated violently, 

 but gave a beautiful green light instead of the violet light given by 

 the other coloured specimens. 



All the fluor-spars showed a green glow when under the action 

 of radium, and a violet glow in the cathode rays. The glow con- 

 tinues in each case for a short time after removal of the exciting 

 source. 



The colour changes produced by radium on fluor-spars are so 

 varied as to be somewhat bewildering. 



A green crystal heated until all the colour was discharged and 

 then given an exposure of two days to 25 mg. of radium showed a 

 marked green colour. On further exposing for four days, this green 

 colour was strengthened up to its original depth, and still longer 

 exposure did not alter this. This result seems to indicate that the 

 colour-producing substance in the green crystals, whatever that 

 may be, is limited in quantity. 



A yellow crystal treated in the same way did not recover its 

 original colour but acquired a peculiar shade of blue, which slowly 

 changed to purple after two months without further treatment. 



1 The distinction between violet and purple appears to be only one of degree 

 and not of actual colour. A small violet crystal appears of the same colour 

 as a large purple one when viewed by transmitted light. Many of the violet 

 or purple crystals are only coloured on the surface or on thin plates or zones 

 within the crystal, the main bulk of the crystal being nearly colourless. This 

 zoning of the colour needs careful inspection to detect and is easily overlooked. 



